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Deposition of DLC film with adhesive W-DLC layer on stainless steel and its tribological properties
Diamond & Related Materials 18 (2009) 1023–1027
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Deposition of DLC film with adhesive W-DLC layer on stainless steel and its tribological properties Takanori Takeno a, Toshifumi Sugawara b, Hiroyuki Miki c, Toshiyuki Takagi c,⁎ a b c
Institute for International Advanced Interdisciplinary Research, Tohoku University International Advanced Research and Education Organization, Japan Graduate School of Engineering, Tohoku University, Japan Institute of Fluid Science, Tohoku University, Japan
a r t i c l e
i n f o
Available online 24 January 2009 Keywords: DLC Metal-doped DLC Friction Double-layered structure
a b s t r a c t Tribological properties of a diamond-like carbon (DLC) coating with an adhesive tungsten-containing DLC (W-DLC) layer were investigated. The coatings were deposited onto AISI316L steel substrates and Si wafers using plasma enhanced chemical vapor deposition and tungsten co-sputtering of the metal target. Methane and argon gases were used as the precursor of the coatings. In this study, three types of coatings were evaluated: DLC/W-DLC on AISI316L (DLC-1), DLC/W-DLC on Si wafer (DLC-2), and DLC on Si wafer (DLC-3). The structural characterizations were performed by transmission electron microscopy and tapping mode atomic force microscopy. At the boundary between the W-DLC layer and the AISI316L substrate, microscopic decohesion or delamination was not observed. The surface roughness of the DLC-1 coating was greater than that of the DLC-2 coating. This feature was derived from the surface roughness of the initial surface of the AISI316L substrate. Friction tests were performed using a rotation-type ball-on-flat configuration tribometer. The observed friction of the DLC-1 coating was unstable compared with the DLC-2 or DLC-3 coatings. This was due to wear debris which had risen to the friction surface resulting in unstable friction on the DLC-1 coating. During the friction studies, the top DLC layer was removed from the adhesive W-DLC layer because the adhesive strength at this part was not enough. In order to achieve the low and stable friction of the DLC coating with the W-DLC layer on AISI316L, it is necessary to improve the smoothness of the surface and the adhesion between the DLC coating and the W-DLC layer. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Many attempts have been made to achieve a good adherence between DLC coatings and steel substrates, since DLC coatings have a wide range of applications. The major techniques to obtain a good adhesive strength include the fabrication of an intermediate ‘glue’ layer between the coating and substrates, and the reduction of the internal stress of the coating. Resent developments have been mainly focused on the former case. Intermediate metal or compound layers such as Ti, Zr, W, Nb, or WC have shown potential to improve the adhesive strength [1]. However, even if an intermediate layer is fabricated, it does not work as an adhesive layer for DLCs deposited by the CVD technique. Wei et al. systematically investigated the adhesiveness of DLC coatings on Si substrates, sputter-deposited Cr on Si, and sputter-deposited Ti on Si [2]. Although the DLC coating could be deposited on the Si substrate without a metallic intermediate, the DLC coatings on sputterdeposited metal intermediates were readily delaminated. This feature ⁎ Corresponding author. Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel./fax: +81 22 217 5248. E-mail address: [email protected] (T. Takagi). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.01.029
was also confirmed by our own experiments on a sputtered W layer as the intermediate layer. The sudden delamination or peeling of the DLC coating was observed just after the deposition. Possibly, the stress level of the DLC coatings deposited by CVD is higher than that deposited by other techniques, and the metal intermediate layer does not have enough strength to adhere the DLCs with high residual stress. Thus a reduction of the residual stress of the DLC coating is one of the important issues in obtaining a good adherence to the metallic substrates. The residual stress of the DLC coating has been reduced by including additional elements. Both light and heavy elements have been considered as dopants. Ban et al. investigated silicon as a dopant for the reduction of the compressive stress in the DLC coating [3]. Their group used SiH4 gas as a precursor of the silicon element. The stress reduction from 2.5 GPa to 1.0 GPa was achieved when the SiH4 flow ratio was 36.4%. Another work was performed by Rabbani et al. on N2-doped a-C:H coatings [4]. Their group also doped nitrogen from the gas phase and investigated how the degree of stress was reduced in view of the nitrogen and argon concentration in the coatings. The most stressful coating was obtained in the case of the Ar inert gas (1.6 GPa), and the minimum stress level (~0.9 GPa) of the coating was achieved by a certain mixture ratio of Ar and N2 gases. Therefore,
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Table 1 Deposition parameters for the coatings investigated in this study. DLC/W-DLC on AISI316L Sample ID Substrate Initial pressure
Gas Substrate self bias Operation pressure Gas Substrate self bias Operation pressure Sputtered target Target DC power
Gas Substrate self bias Operation pressure
DLC-1 AISI316L
DLC/W-DLC on Si
DLC on Si
DLC-2 DLC-3 Single crystal Si wafer −4 4.0 × 10 Pa
Pretreatment of the substrates Ar = 10.0 sccm − 400 V 3.3 Pa Deposition of the adhesive layer (W-DLC) Ar/CH4 = 10.0/3.0 − 400 V 3.3 Pa Tungsten (W) 40 W
– – – – –
Deposition of the DLC layer CH4 = 10.0 sccm − 400 V 3.3 Pa
doping of additional elements into the DLC matrix is one of the ways to reduce the residual stress of the coatings. The inclusion of metal nanoclusters into the DLC matrix is also a good candidate. Our previous reports have shown that metalcontaining DLC (Me-DLC) coatings can be made by the hybrid deposition technique of CVD and PVD [5,6]. The structure of the coating is defined by the metal clusters of several nanometer-sizes which are dispersed into the amorphous carbon host matrix. The inclusion of metal clusters into the DLC coating is expected to reduce the residual stress of the coating, which may result in the successful deposition of metallic substrates without an intermediate layer. Even if a Me-DLC coating can be deposited onto a metal substrate, there is another issue to be solved. Concerning tribological properties, Me-DLC coatings sometimes show high wear as the number of friction cycles increases [7]. The friction behavior of the W-DLC layers against the AISI304 stainless steel ball shows that a drastic increase in friction coefficient occurred in the early stage of friction cycling, and the coatings were worn by the sliding. However, double-layered structures like DLC/W-DLC on Si show excellent friction behavior, and the average friction coefficient in ambient nitrogen was under 0.1. Thus this double-layered structure on stainless steel substrates is a very promising combination in terms of industrial application. Thus our concept is that the W-DLC layer is used as an intermediate and the DLC coating is used as a coating showing good tribological performance. In this study, a DLC coating with an adhesive W-DLC layer deposited onto AISI316L stainless steel substrates was investigated. Experiments were performed for three types of coatings; DLC/W-DLC on AISI316L, DLC/W-DLC on Si wafer, and DLC on Si wafer. The crosssectional microstructure and surface morphology of the coatings was analyzed. Then, friction tests were carried out. The tribological properties of the coatings are discussed in terms of the cross-sectional structure and surface morphology of the coatings.
target. The parameters used in this study were determined from our preliminary results. The deposition procedure is as follows. Substrates were evacuated up to 4.0 × 10− 4 Pa. Before the deposition, the substrates were cleaned by RF-discharged Ar plasma in order to remove surface contamination. The investigated coatings were fabricated onto AISI316L stainless steel in two steps. First, the W-DLC adhesive layer was fabricated onto the well-polished substrates by RF discharged methane plasma and DC magnetron co-sputtering of a tungsten metal target. Secondary, a DLC coating was deposited onto the W-DLC adhesive layer using methane plasma. The detailed deposition parameters used in this study are summarized in Table 1. In this study, three types of coatings were investigated for the tribological properties. The boundary structure between the adhesive layer and the metal substrate was analyzed by transmission electron microscopy (TEM) using a JEOL JEM-3010 instrument under an acceleration voltage of 300 kV. The sample for TEM observation was prepared by an ion slicer using a JEOL EM-09100IS instrument. Friction tests were performed at room temperature using a rotation type ball-on-disk type tribometer manufactured by CSM Instruments. A ball of 6 mm in diameter made of AISI304 stainless steel was used as a counter material. The normal load of 1 N was applied to the ball and the linear sliding speed was set at 12 cm/s. The testing apparatus was enclosed within an acrylic chamber. The chamber was first evacuated and then, dry air or nitrogen was introduced into the chamber. The friction coefficient was measured in two types of ambient conditions: in air with a well-controlled humidity (40 RH%), and in a nitrogen atmosphere condition (RH b 10%). The humidity during the friction test was monitored using a commercial hygrometer and the fluctuation was within 5%, as measured from the start of the experiment. After the friction test, the surface of the ball was analyzed by using Scanning Electron Microscope equipped with Energy Dispersive X-ray spectrometer (Hitachi High-Technologies S-4700 Scanning Electron Microscope). 3. Results and discussions 3.1. Characterization of the coatings Fig. 1 shows the cross-sectional TEM image of the boundary structure between the W-DLC adhesive layer and AISI316L substrate. The thickness of the adhesive layer was about 1 μm. No microscopic
2. Experimental procedure The coatings were deposited by plasma-enhanced chemical vapor deposition technique and DC magnetron co-sputtering of tungsten metal target. The DC magnetron sputter locates upside of the deposition chamber. The substrate holder is at the bottom of it, and the RF power is applied to the holder. The distance between the target and the holder is fixed as 100 mm. The nozzle end of argon gas is faced to the target and the gas blows against it. On the other hand, methane is introduced from the backside of the deposition chamber. These techniques allow us to avoid the possible poisoning of the tungsten
Fig. 1. Cross-sectional transmission electron micrograph of the W-DLC adhesive layer on a AISI316L substrate.
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size of the tungsten clusters and the degree of dispersiveness in the amorphous carbon host matrix. But, once the surface of the W-DLC layer is covered with DLC coating, the surface roughness of the final form of the coating decreases compared with the single W-DLC layer [7]. The DLC coating grows as filling the bumpy surface of the W-DLC layer. However, the surface roughness of the DLC-1 coating is greater than that of the DLC-2 coating. Assuming that the same DLC coating and W-DLC adhesive layer were fabricated even if the substrate material is different, the rough surface in the DLC-1 coating may be derived from the initial surface morphology of the AISI316L substrate. 3.2. Tribological properties The ball-on-flat configuration has been used to investigate the tribological properties of three types of coatings. The results of the friction tests are shown in Fig. 3. The DLC-1 sample (Fig. 3(a)) exhibited an average friction coefficient within the range of 0.15–0.20 under nitrogen condition after the running-in process. In the case of air condition, an unstable friction coefficient was observed as the number increases. The friction coefficient of the DLC-2 sample showed a stable behavior (Fig. 3(b)) in both air and nitrogen atmosphere condition. After the running-in process, the average friction coefficient in the steady-state regime was 0.07. The third set of experimental results of DLC-3 (Fig. 3(c)) showed a gradual increase in the friction coefficient. After the 40 × 103 cycles, the friction was very noisy. Fig. 4 shows a photograph of the wear tracks and the balls of the DLC-1 and DLC-2 coatings at the contact point after the friction test. In
Fig. 2. Surface morphology of the coatings: (a) DLC-1 and (b) DLC-2. The calculated surface roughness (Ra) of the DLC-1 and the DLC-2 coatings were 1.675 nm and 0.487 nm, respectively.
decohesion was observed between the layer and substrate. The crystallographic structure of the W-DLC layer indicates that tungsten metal clusters of several nanometers in size are well dispersed in the amorphous carbon host matrix, which was confirmed by the magnified TEM image [6]. The dispersive metal clusters lead to a reduction of the stress in the DLC coating and helped to achieve a good adherence to the stainless steel substrates. The successful deposition of the W-DLC intermediate layer was confirmed by adding the tungsten metal clusters into the amorphous carbon host matrix. The surface morphologies of the DLC-1 and DLC-2 coatings were obtained by tapping mode AFM and the results are shown in Fig. 2. The images were obtained in 10 × 10 mm2 in area. The surface roughness of the DLC-1 and DLC-2 coatings was calculated as Ra = 1.675 nm and Ra = 0.487, respectively. After the deposition of the W-DLC layer, a rather rough surface was obtained, which resulted from the average
Fig. 3. Friction behavior of three types of coating: (a) DLC-1, (b) DLC-2 and (c) DLC-3. Red and blue lines represent the results in the case of air and nitrogen atmosphere condition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Optical images of the balls and the wear tracks: (a) ball and (b) wear track of the DLC1 coating, and (c) ball and (d) wear track of the DLC-2 coating after a friction test in nitrogen atmosphere condition.
order to avoid the oxygen in air for the friction behavior, these pictures show the results of the nitrogen condition. The sizes of the transfer films on the balls were approximately 130 μm in both cases, and the wear tracks of both cases were shown to have almost the same size. In addition to that, wear debris was observed on the surface along the wear track. Although the generation of debris was observed to have occurred, no ruptures inside the wear track were observed in either sample. After the friction tests, the ball surface was analyzed by the SEM equipped with Energy Dispersive X-ray Spectrometer. Fig. 5 shows the surface image and elemental mapping results on the ball after the 100,000 cycles in ambient air condition on DLC-1. One can see that transferred film contains carbon, tungsten and oxygen. Observed carbon element come from the top DLC layer and the W-DLC adhesive layer, and tungsten and oxygen elements come from the W-DLC layer
and ambient air, respectively. These results indicate that the coating was worn and the ball surface reached to the adhesive W-DLC layer. In addition to that, it can be considered that tungsten react with the oxygen and form the tungsten oxide debris. Focusing on the effect of the surface roughness on the friction coefficient, its contribution was considered to be large. The rough surface led to the high friction in the W-DLC coating and a minimum friction coefficient of ~0.1 was obtained in Ra = 7 nm in air condition after 100,000 cycles [8]. The same feature was also shown in this paper for the Cr-DLC coatings. Ohana et al., reported on the effect of the surface roughness of the DLC coating on a steel substrate [9]. They investigated the friction tests in water. The minimum friction coefficient was achieved in Ra = 34 nm. Their group also investigated the coating deposited onto a smoother surface (Ra = 2 nm), but the measured friction coefficient was somewhat higher compared with
Fig. 5. SEM/EDX results on the ball after the 100,000 cycle friction test in ambient air condition of DLC-1. The images were obtained in 800 magnification.
T. Takeno et al. / Diamond & Related Materials 18 (2009) 1023–1027
the result with Ra = 34 nm. These results indicated that the surface roughness and the test environment are key factors for low friction, where low friction can be achieved when the coating surface is smooth. In our case, the friction tests were carried out in a gaseous atmosphere and low friction was achieved in the DLC-2 coating. The results agreed with the above-mentioned papers for the surface roughness of the DLC-2 coating. However, the friction coefficient of the DLC-1 coating was unstable. In case of the unstable friction shown in Fig. 3(a), the behavior can be due to the “rolling effect” of brittle and hard tungsten oxides. The idea as a possible mechanism comes from the consideration as follows. After the ball reached to the W-DLC layer, the tungsten in the W-DLC layer could react chemically with the oxygen in ambient air during the friction, and the tungsten oxides was formed. The oxides came to the contact point easily because of the rough surface, and were inserted into the contact point. The debris possibly played a role like a roller. However, the debris was sometimes come out from the wear track. By continuous insertion and exclusion, unstable friction behavior observed. On the other hand, stable friction shown in Fig. 3(b) is due to the smooth surface of the coating. Possibly, the ball also approached to the W-DLC layer. However, the debris may not be inserted in the contact point because of the very smooth surface. The debris was just come out from the wear track and could not be inserted into the contact point even if the oxides were formed. Thus, it can be considered that the friction behavior is stable since no roller effect is available. The above consideration seems reasonable when we focus on the amount of the wear debris at the edge of the wear track. From the optical photographs shown in Fig. 4(b) and (d), large amount of the wear debris was observed in case of DLC-2 compared to that of DLC-1. The results also could be the evidence for the above mechanisms. Thus, the increase in adhesive strength and the decrease in surface roughness are required for the low and stable friction. From the discussion above, the decrease in the surface roughness and the increase in the adhesive strength are important for the low friction. 4. Summary In this study, a DLC coating with an adhesive W-DLC intermediate was deposited onto steel substrates and its tribological properties
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investigated. The adhesive W-DLC intermediate was successfully deposited onto an AISI316L substrate and no microscopic decohesion or delamination was observed. The surface roughness of the DLC coating with adhesive intermediate was greater than that of the coating deposited onto a Si substrate. The rough surface was derived from the surface roughness of the AISI316L substrate. The tribological property of the DLC-1 coating showed variation of the friction coefficient. Such an unstable friction was caused by wear debris on the surface. Comparing the friction behavior of the DLC-1 coating with the previous reports and the data of the DLC-2 coating, low friction can be realized in the case of coatings with a smoother surface and strong adhesive strength between the DLC coating and the adhesive W-DLC layer. Acknowledgments This work was partly supported by a Grant-in-Aid for Young Scientists (B) (No.20760095) and a Grant-in-Aid for Scientific Research (B) (20360380) of Japan Society for the Promotion of Science (JSPS). The authors express their great appreciation to Mr. Takeshi Sato from the Institute of Fluid Science at Tohoku University for his technical assistance. References [1] Y. Pauleau, Residual stresses in DLC films and adhesion to various substrates, in: C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-Like Carbon Films, Springer, 2008. [2] C.H. Wei, J.Y. Yen, Diamond and Related Materials 16 (2007) 1325. [3] M. Ban, T. Hasegawa, Surface & Coatings Technology 162 (2003) 1. [4] F. Rabbani, R.E. Galindo, W.M. Arnoldbik, S. van der Zwaag, A. van Veen, H. Schut, Diamond and Related Materials 13 (2004) 1645. [5] T. Takeno, H. Miki, T. Sugawara, Y. Hoshi, T. Takagi, Diamond and Related Materials 17 (2008) 713. [6] T. Takeno, H. Miki, T. Takagi, H. Onodera, Diamond and Related Materials 15 (2006) 1902. [7] H. Miki, T. Takeno, T. Takagi, Thin Solid Films 516 (2008) 5414. [8] F. Svahn, A. Kassman-Rudolphi, E. Wallen, Wear 254 (2003) 1092. [9] T. Ohana, M. Suzuki, T. Nakamura, A. Tanaka, Y. Koga, Diamond and Related Materials 13 (2004) 2211.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Deposition of DLC film with adhesive W-DLC layer on stainless steel and its tribological properties Takanori Takeno a, Toshifumi Sugawara b, Hiroyuki Miki c, Toshiyuki Takagi c,⁎ a b c
Institute for International Advanced Interdisciplinary Research, Tohoku University International Advanced Research and Education Organization, Japan Graduate School of Engineering, Tohoku University, Japan Institute of Fluid Science, Tohoku University, Japan
a r t i c l e
i n f o
Available online 24 January 2009 Keywords: DLC Metal-doped DLC Friction Double-layered structure
a b s t r a c t Tribological properties of a diamond-like carbon (DLC) coating with an adhesive tungsten-containing DLC (W-DLC) layer were investigated. The coatings were deposited onto AISI316L steel substrates and Si wafers using plasma enhanced chemical vapor deposition and tungsten co-sputtering of the metal target. Methane and argon gases were used as the precursor of the coatings. In this study, three types of coatings were evaluated: DLC/W-DLC on AISI316L (DLC-1), DLC/W-DLC on Si wafer (DLC-2), and DLC on Si wafer (DLC-3). The structural characterizations were performed by transmission electron microscopy and tapping mode atomic force microscopy. At the boundary between the W-DLC layer and the AISI316L substrate, microscopic decohesion or delamination was not observed. The surface roughness of the DLC-1 coating was greater than that of the DLC-2 coating. This feature was derived from the surface roughness of the initial surface of the AISI316L substrate. Friction tests were performed using a rotation-type ball-on-flat configuration tribometer. The observed friction of the DLC-1 coating was unstable compared with the DLC-2 or DLC-3 coatings. This was due to wear debris which had risen to the friction surface resulting in unstable friction on the DLC-1 coating. During the friction studies, the top DLC layer was removed from the adhesive W-DLC layer because the adhesive strength at this part was not enough. In order to achieve the low and stable friction of the DLC coating with the W-DLC layer on AISI316L, it is necessary to improve the smoothness of the surface and the adhesion between the DLC coating and the W-DLC layer. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Many attempts have been made to achieve a good adherence between DLC coatings and steel substrates, since DLC coatings have a wide range of applications. The major techniques to obtain a good adhesive strength include the fabrication of an intermediate ‘glue’ layer between the coating and substrates, and the reduction of the internal stress of the coating. Resent developments have been mainly focused on the former case. Intermediate metal or compound layers such as Ti, Zr, W, Nb, or WC have shown potential to improve the adhesive strength [1]. However, even if an intermediate layer is fabricated, it does not work as an adhesive layer for DLCs deposited by the CVD technique. Wei et al. systematically investigated the adhesiveness of DLC coatings on Si substrates, sputter-deposited Cr on Si, and sputter-deposited Ti on Si [2]. Although the DLC coating could be deposited on the Si substrate without a metallic intermediate, the DLC coatings on sputterdeposited metal intermediates were readily delaminated. This feature ⁎ Corresponding author. Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel./fax: +81 22 217 5248. E-mail address: [email protected] (T. Takagi). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.01.029
was also confirmed by our own experiments on a sputtered W layer as the intermediate layer. The sudden delamination or peeling of the DLC coating was observed just after the deposition. Possibly, the stress level of the DLC coatings deposited by CVD is higher than that deposited by other techniques, and the metal intermediate layer does not have enough strength to adhere the DLCs with high residual stress. Thus a reduction of the residual stress of the DLC coating is one of the important issues in obtaining a good adherence to the metallic substrates. The residual stress of the DLC coating has been reduced by including additional elements. Both light and heavy elements have been considered as dopants. Ban et al. investigated silicon as a dopant for the reduction of the compressive stress in the DLC coating [3]. Their group used SiH4 gas as a precursor of the silicon element. The stress reduction from 2.5 GPa to 1.0 GPa was achieved when the SiH4 flow ratio was 36.4%. Another work was performed by Rabbani et al. on N2-doped a-C:H coatings [4]. Their group also doped nitrogen from the gas phase and investigated how the degree of stress was reduced in view of the nitrogen and argon concentration in the coatings. The most stressful coating was obtained in the case of the Ar inert gas (1.6 GPa), and the minimum stress level (~0.9 GPa) of the coating was achieved by a certain mixture ratio of Ar and N2 gases. Therefore,
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T. Takeno et al. / Diamond & Related Materials 18 (2009) 1023–1027
Table 1 Deposition parameters for the coatings investigated in this study. DLC/W-DLC on AISI316L Sample ID Substrate Initial pressure
Gas Substrate self bias Operation pressure Gas Substrate self bias Operation pressure Sputtered target Target DC power
Gas Substrate self bias Operation pressure
DLC-1 AISI316L
DLC/W-DLC on Si
DLC on Si
DLC-2 DLC-3 Single crystal Si wafer −4 4.0 × 10 Pa
Pretreatment of the substrates Ar = 10.0 sccm − 400 V 3.3 Pa Deposition of the adhesive layer (W-DLC) Ar/CH4 = 10.0/3.0 − 400 V 3.3 Pa Tungsten (W) 40 W
– – – – –
Deposition of the DLC layer CH4 = 10.0 sccm − 400 V 3.3 Pa
doping of additional elements into the DLC matrix is one of the ways to reduce the residual stress of the coatings. The inclusion of metal nanoclusters into the DLC matrix is also a good candidate. Our previous reports have shown that metalcontaining DLC (Me-DLC) coatings can be made by the hybrid deposition technique of CVD and PVD [5,6]. The structure of the coating is defined by the metal clusters of several nanometer-sizes which are dispersed into the amorphous carbon host matrix. The inclusion of metal clusters into the DLC coating is expected to reduce the residual stress of the coating, which may result in the successful deposition of metallic substrates without an intermediate layer. Even if a Me-DLC coating can be deposited onto a metal substrate, there is another issue to be solved. Concerning tribological properties, Me-DLC coatings sometimes show high wear as the number of friction cycles increases [7]. The friction behavior of the W-DLC layers against the AISI304 stainless steel ball shows that a drastic increase in friction coefficient occurred in the early stage of friction cycling, and the coatings were worn by the sliding. However, double-layered structures like DLC/W-DLC on Si show excellent friction behavior, and the average friction coefficient in ambient nitrogen was under 0.1. Thus this double-layered structure on stainless steel substrates is a very promising combination in terms of industrial application. Thus our concept is that the W-DLC layer is used as an intermediate and the DLC coating is used as a coating showing good tribological performance. In this study, a DLC coating with an adhesive W-DLC layer deposited onto AISI316L stainless steel substrates was investigated. Experiments were performed for three types of coatings; DLC/W-DLC on AISI316L, DLC/W-DLC on Si wafer, and DLC on Si wafer. The crosssectional microstructure and surface morphology of the coatings was analyzed. Then, friction tests were carried out. The tribological properties of the coatings are discussed in terms of the cross-sectional structure and surface morphology of the coatings.
target. The parameters used in this study were determined from our preliminary results. The deposition procedure is as follows. Substrates were evacuated up to 4.0 × 10− 4 Pa. Before the deposition, the substrates were cleaned by RF-discharged Ar plasma in order to remove surface contamination. The investigated coatings were fabricated onto AISI316L stainless steel in two steps. First, the W-DLC adhesive layer was fabricated onto the well-polished substrates by RF discharged methane plasma and DC magnetron co-sputtering of a tungsten metal target. Secondary, a DLC coating was deposited onto the W-DLC adhesive layer using methane plasma. The detailed deposition parameters used in this study are summarized in Table 1. In this study, three types of coatings were investigated for the tribological properties. The boundary structure between the adhesive layer and the metal substrate was analyzed by transmission electron microscopy (TEM) using a JEOL JEM-3010 instrument under an acceleration voltage of 300 kV. The sample for TEM observation was prepared by an ion slicer using a JEOL EM-09100IS instrument. Friction tests were performed at room temperature using a rotation type ball-on-disk type tribometer manufactured by CSM Instruments. A ball of 6 mm in diameter made of AISI304 stainless steel was used as a counter material. The normal load of 1 N was applied to the ball and the linear sliding speed was set at 12 cm/s. The testing apparatus was enclosed within an acrylic chamber. The chamber was first evacuated and then, dry air or nitrogen was introduced into the chamber. The friction coefficient was measured in two types of ambient conditions: in air with a well-controlled humidity (40 RH%), and in a nitrogen atmosphere condition (RH b 10%). The humidity during the friction test was monitored using a commercial hygrometer and the fluctuation was within 5%, as measured from the start of the experiment. After the friction test, the surface of the ball was analyzed by using Scanning Electron Microscope equipped with Energy Dispersive X-ray spectrometer (Hitachi High-Technologies S-4700 Scanning Electron Microscope). 3. Results and discussions 3.1. Characterization of the coatings Fig. 1 shows the cross-sectional TEM image of the boundary structure between the W-DLC adhesive layer and AISI316L substrate. The thickness of the adhesive layer was about 1 μm. No microscopic
2. Experimental procedure The coatings were deposited by plasma-enhanced chemical vapor deposition technique and DC magnetron co-sputtering of tungsten metal target. The DC magnetron sputter locates upside of the deposition chamber. The substrate holder is at the bottom of it, and the RF power is applied to the holder. The distance between the target and the holder is fixed as 100 mm. The nozzle end of argon gas is faced to the target and the gas blows against it. On the other hand, methane is introduced from the backside of the deposition chamber. These techniques allow us to avoid the possible poisoning of the tungsten
Fig. 1. Cross-sectional transmission electron micrograph of the W-DLC adhesive layer on a AISI316L substrate.
T. Takeno et al. / Diamond & Related Materials 18 (2009) 1023–1027
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size of the tungsten clusters and the degree of dispersiveness in the amorphous carbon host matrix. But, once the surface of the W-DLC layer is covered with DLC coating, the surface roughness of the final form of the coating decreases compared with the single W-DLC layer [7]. The DLC coating grows as filling the bumpy surface of the W-DLC layer. However, the surface roughness of the DLC-1 coating is greater than that of the DLC-2 coating. Assuming that the same DLC coating and W-DLC adhesive layer were fabricated even if the substrate material is different, the rough surface in the DLC-1 coating may be derived from the initial surface morphology of the AISI316L substrate. 3.2. Tribological properties The ball-on-flat configuration has been used to investigate the tribological properties of three types of coatings. The results of the friction tests are shown in Fig. 3. The DLC-1 sample (Fig. 3(a)) exhibited an average friction coefficient within the range of 0.15–0.20 under nitrogen condition after the running-in process. In the case of air condition, an unstable friction coefficient was observed as the number increases. The friction coefficient of the DLC-2 sample showed a stable behavior (Fig. 3(b)) in both air and nitrogen atmosphere condition. After the running-in process, the average friction coefficient in the steady-state regime was 0.07. The third set of experimental results of DLC-3 (Fig. 3(c)) showed a gradual increase in the friction coefficient. After the 40 × 103 cycles, the friction was very noisy. Fig. 4 shows a photograph of the wear tracks and the balls of the DLC-1 and DLC-2 coatings at the contact point after the friction test. In
Fig. 2. Surface morphology of the coatings: (a) DLC-1 and (b) DLC-2. The calculated surface roughness (Ra) of the DLC-1 and the DLC-2 coatings were 1.675 nm and 0.487 nm, respectively.
decohesion was observed between the layer and substrate. The crystallographic structure of the W-DLC layer indicates that tungsten metal clusters of several nanometers in size are well dispersed in the amorphous carbon host matrix, which was confirmed by the magnified TEM image [6]. The dispersive metal clusters lead to a reduction of the stress in the DLC coating and helped to achieve a good adherence to the stainless steel substrates. The successful deposition of the W-DLC intermediate layer was confirmed by adding the tungsten metal clusters into the amorphous carbon host matrix. The surface morphologies of the DLC-1 and DLC-2 coatings were obtained by tapping mode AFM and the results are shown in Fig. 2. The images were obtained in 10 × 10 mm2 in area. The surface roughness of the DLC-1 and DLC-2 coatings was calculated as Ra = 1.675 nm and Ra = 0.487, respectively. After the deposition of the W-DLC layer, a rather rough surface was obtained, which resulted from the average
Fig. 3. Friction behavior of three types of coating: (a) DLC-1, (b) DLC-2 and (c) DLC-3. Red and blue lines represent the results in the case of air and nitrogen atmosphere condition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Optical images of the balls and the wear tracks: (a) ball and (b) wear track of the DLC1 coating, and (c) ball and (d) wear track of the DLC-2 coating after a friction test in nitrogen atmosphere condition.
order to avoid the oxygen in air for the friction behavior, these pictures show the results of the nitrogen condition. The sizes of the transfer films on the balls were approximately 130 μm in both cases, and the wear tracks of both cases were shown to have almost the same size. In addition to that, wear debris was observed on the surface along the wear track. Although the generation of debris was observed to have occurred, no ruptures inside the wear track were observed in either sample. After the friction tests, the ball surface was analyzed by the SEM equipped with Energy Dispersive X-ray Spectrometer. Fig. 5 shows the surface image and elemental mapping results on the ball after the 100,000 cycles in ambient air condition on DLC-1. One can see that transferred film contains carbon, tungsten and oxygen. Observed carbon element come from the top DLC layer and the W-DLC adhesive layer, and tungsten and oxygen elements come from the W-DLC layer
and ambient air, respectively. These results indicate that the coating was worn and the ball surface reached to the adhesive W-DLC layer. In addition to that, it can be considered that tungsten react with the oxygen and form the tungsten oxide debris. Focusing on the effect of the surface roughness on the friction coefficient, its contribution was considered to be large. The rough surface led to the high friction in the W-DLC coating and a minimum friction coefficient of ~0.1 was obtained in Ra = 7 nm in air condition after 100,000 cycles [8]. The same feature was also shown in this paper for the Cr-DLC coatings. Ohana et al., reported on the effect of the surface roughness of the DLC coating on a steel substrate [9]. They investigated the friction tests in water. The minimum friction coefficient was achieved in Ra = 34 nm. Their group also investigated the coating deposited onto a smoother surface (Ra = 2 nm), but the measured friction coefficient was somewhat higher compared with
Fig. 5. SEM/EDX results on the ball after the 100,000 cycle friction test in ambient air condition of DLC-1. The images were obtained in 800 magnification.
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the result with Ra = 34 nm. These results indicated that the surface roughness and the test environment are key factors for low friction, where low friction can be achieved when the coating surface is smooth. In our case, the friction tests were carried out in a gaseous atmosphere and low friction was achieved in the DLC-2 coating. The results agreed with the above-mentioned papers for the surface roughness of the DLC-2 coating. However, the friction coefficient of the DLC-1 coating was unstable. In case of the unstable friction shown in Fig. 3(a), the behavior can be due to the “rolling effect” of brittle and hard tungsten oxides. The idea as a possible mechanism comes from the consideration as follows. After the ball reached to the W-DLC layer, the tungsten in the W-DLC layer could react chemically with the oxygen in ambient air during the friction, and the tungsten oxides was formed. The oxides came to the contact point easily because of the rough surface, and were inserted into the contact point. The debris possibly played a role like a roller. However, the debris was sometimes come out from the wear track. By continuous insertion and exclusion, unstable friction behavior observed. On the other hand, stable friction shown in Fig. 3(b) is due to the smooth surface of the coating. Possibly, the ball also approached to the W-DLC layer. However, the debris may not be inserted in the contact point because of the very smooth surface. The debris was just come out from the wear track and could not be inserted into the contact point even if the oxides were formed. Thus, it can be considered that the friction behavior is stable since no roller effect is available. The above consideration seems reasonable when we focus on the amount of the wear debris at the edge of the wear track. From the optical photographs shown in Fig. 4(b) and (d), large amount of the wear debris was observed in case of DLC-2 compared to that of DLC-1. The results also could be the evidence for the above mechanisms. Thus, the increase in adhesive strength and the decrease in surface roughness are required for the low and stable friction. From the discussion above, the decrease in the surface roughness and the increase in the adhesive strength are important for the low friction. 4. Summary In this study, a DLC coating with an adhesive W-DLC intermediate was deposited onto steel substrates and its tribological properties
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investigated. The adhesive W-DLC intermediate was successfully deposited onto an AISI316L substrate and no microscopic decohesion or delamination was observed. The surface roughness of the DLC coating with adhesive intermediate was greater than that of the coating deposited onto a Si substrate. The rough surface was derived from the surface roughness of the AISI316L substrate. The tribological property of the DLC-1 coating showed variation of the friction coefficient. Such an unstable friction was caused by wear debris on the surface. Comparing the friction behavior of the DLC-1 coating with the previous reports and the data of the DLC-2 coating, low friction can be realized in the case of coatings with a smoother surface and strong adhesive strength between the DLC coating and the adhesive W-DLC layer. Acknowledgments This work was partly supported by a Grant-in-Aid for Young Scientists (B) (No.20760095) and a Grant-in-Aid for Scientific Research (B) (20360380) of Japan Society for the Promotion of Science (JSPS). The authors express their great appreciation to Mr. Takeshi Sato from the Institute of Fluid Science at Tohoku University for his technical assistance. References [1] Y. Pauleau, Residual stresses in DLC films and adhesion to various substrates, in: C. Donnet, A. Erdemir (Eds.), Tribology of Diamond-Like Carbon Films, Springer, 2008. [2] C.H. Wei, J.Y. Yen, Diamond and Related Materials 16 (2007) 1325. [3] M. Ban, T. Hasegawa, Surface & Coatings Technology 162 (2003) 1. [4] F. Rabbani, R.E. Galindo, W.M. Arnoldbik, S. van der Zwaag, A. van Veen, H. Schut, Diamond and Related Materials 13 (2004) 1645. [5] T. Takeno, H. Miki, T. Sugawara, Y. Hoshi, T. Takagi, Diamond and Related Materials 17 (2008) 713. [6] T. Takeno, H. Miki, T. Takagi, H. Onodera, Diamond and Related Materials 15 (2006) 1902. [7] H. Miki, T. Takeno, T. Takagi, Thin Solid Films 516 (2008) 5414. [8] F. Svahn, A. Kassman-Rudolphi, E. Wallen, Wear 254 (2003) 1092. [9] T. Ohana, M. Suzuki, T. Nakamura, A. Tanaka, Y. Koga, Diamond and Related Materials 13 (2004) 2211.